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Correlations between ground observations of Pi 2 geomagnetic pulsations and satellite plasma density observations
0032XI633/83/020143-18$03.00/O 0 1983 Per~aamon Press Ltd.
Phet.SpoceScr., Vol. 31,No.Z,pp.143~160,1983 Printed m Great Britain.
CORRELATIONS BETWEEN GROUND OBSERVATIONS OF Pi 2 GEOMAGNETIC PULSATIONS AND SATELLITE PLASMA DENSITY OBSERVATIONS* Department
M. LESTER? and D. ORR of Physics, University of York, Heslington, (Received
infinulform
York, YOl SDD, U.K.
5 July 1982)
Abstract -Ground observations of Pi 2 geomagnetic pulsations are correlated with satellite measurements of plasma density for three time intervals. The pulsations were recorded using the IGS network of magnetometer stations and the plasma density measurements were made on board GEOS-1 and ISEE-1. Using the technique of complex demodulation, the amplitude, phase and polarisation characteristics of the Pi 2 pulsations are observed along two meridional profiles ; one from Eidar, Iceland (L = 6.7) to Cambridge, U.K. (L = 2.5) and the other from Tromso, Norway (L = 6.2) to Nurmijarvi, Finland (L = 3.3). The observed characteristics of the Pi 2 pulsations are then compared with the plasma density measurements. Close relationships between the plasmapause position and the position of an elliptic&y reversal and a variation in H component phase are observed. A small, secondary amplitude maximum is observed on the U.K./Iceland meridian well inside the position of the projection of the equatorial plasmapause. The primary maxima on the two meridians, in general occur close to the estimated position of the equatorward edge of a westward electrojet. Using the plasma density measurements, the periods of surface waves at the plasmapause for two intervals are estimated and found to be in good agreement with the dominant spectral peaks observed at the ground stations near the plasmapause latitude and within the plasmasphere. The polarisation reversal, together with phase characteristics, spectral evidence and the agreement between the theoretical and observed periods leads to the suggestion that on occasions a surface wave is excited on the plasmapause as an intermediate stage in the propagation of Pi 2 pulsations from the aurora1 zone to lower latitudes. 1. INTRODUCTION
(Carpenter, 1966), (see Fig. 12 of Lester and Orr (1981)). This type of study has been extended (Stuart and Baransky, 1982) by using two meridional profiles, one in the U.K. and one in Scandinavia. Stuart and Baransky (1982) used a statistically derived position for the plasmapause based on the results of Chappell et al. (1970) and Orr and Webb (1975). Using a very simple analysis method, Stuart and Baransky (1982) inferred that the sense of polarisation and the spectral structure of the Pi 2 changed at the plasmapause, but also that a minimum in the H component amplitude also occurred there. Stuart and Baransky (1982) suggested that Pi 2 pulsations excite a cavity resonance in the plasmasphere. Using an average or a statistically derived plasmapause position does not describe accurately the relationship between the variations of Pi 2 characteristics, in particular the polarisation reversal, and the plasmapause because of the variability of the plasmapause position which is related to substorm activity. In an attempt to obtain a more precise relationship between these two features, we have studied occasions when satellite data are available simultaneously with the ground magnetic signature of Pi 2 pulsations. In particular measurements of plasma density in the near magnetosphere (2 < L < 7) are correlated with the characteristics of Pi 2 pulsations observed on two meridional chains of magnetometers
The latitudinal characteristics of Pi 2 geomagnetic pulsations have been extensively studied using arrays of ground based magnetometers at middle latitudes (e.g. Fukunishi, 1975 ; Stuart et al., 1979; Lester and Orr, 1981) and in the aurora1 zone (Rostoker and Samson, 1981). The main purposes of the mid-latitude studies have been to determine the mode of propagation of Pi 2 pulsations at these latitudes and to identify any characteristics which may be used as diagnostics of the plasmapause. In a recent study, Lester and Orr (1981) showed statistically that there is a reversal in the sense of polarisation at middle latitudes (corresponding typically to L values in the range 3.2-4.5) from clockwise, looking down the field line in the Northern Hemisphere, to anticlockwise as one traverses this middle latitude region from high to low latitudes. This confirmed the earlier work of Fukunishi (1975). The position of the L value of the reversal was observed to have a close correlation with the average position of the plasmapause deduced from whistler observations
*Paper presented at the Fourth IAGA Assembly, Edinburgh, 3-15 August 1981. t Present address : Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, U.S.A. 143
M. LESTERand D.
144
ORR
TABLE1. STATIONS USED
IN THIS PAPER. Atso SHOWN ARE THE SYMBOL, ANDTHE~VALUEANDCORRECTEZDMAGNETICLATITUDEANDLONGITUDE VALUES AT THE EARTH'SSURFACE FOR IGRF(FIRST 7 HARMONICS) 1975
(COMPUTED BYM. R. WARNER) Station
28 0
-
EG RV FA LE DU LL ES YO CA
67.2 66.6 61.7 58.9 58.0 56.2 54.3 52.6 50.6
74.5 71.2 81.3 84.3 80.9 80.4 80.5 81.6 81.8
6.68 6.35 4.46 3.75 3.56 3.23 2.94 2.71 2.48
Tromso Kiruna Oulu Nurmijarvi
TR KI CL NU
66.2 64.3 61.2 56.6
105.4 104.8 106.7 103.6
6.16 5.31 4.31 3.31
EG
H
20 0
component I
I
1
I
28.0
40 0
12 0
I2 0
1 1
1
80
80 I
I T
40
80
1
I
40
80
I
I
40
80
1
I
0.40
L value
Eidar Leirvogour Torshavn Lerwick Durness Loch Laggen Eskdalemuir York Cambridge
nT
i
Corrected magnetic E Longitude Latitude
Symbol
0 45
0 50
0 55
10
FIG. l(a)
nT
Ground-satellite
during three intervals. The plasma density data are taken from the relaxation sounder experiments on board GEOS-1 (Higel, 1978) and ISEE(Etcheto, 1976). The magnetometer data are taken from the IGS/Imperial College/University of York network of stations in the U.K., Iceland and Scandinavia (Table 1). The position of the plasmapause is compared with the latitudinal variations of, not only the polarisation parameters, but also amplitude and phase obtained using the technique of complex demodulation (Beamish et al., 1979; Webb, 1979) and spectral characteristics. TR day 332
145
observations of Pi 2 pulsations 2. THE DATA
(a) Day 332 1977 A Pi 2 pulsation was observed at 0047 U.T. along both the U.K./Iceland and Scandinavian meridional chains (Fig. 1). This is equivalent to - 0047 L.T. for the U.K. meridian and -0215 L.T. for the Scandinavian meridian. From Fig. 1 it can be seen that there appear to be two separate parts to the event, with different periods. Power spectral analysis of these two parts shows that in the first part the dominant period is -70s and in the second - 120s. This is true at all
1977 H component
TR
day 332
1977
D component
T
1
5 OnT
2 OnT
I
I
60
60
I
NU
NU
I 0
i
F1c.1. BAND PASSFILTEKED WAVEFORMS SHOWING A Pi2 PULSATIONOCCURRING ON DAY 332,1977. The filter bandwidth is 5-50 mHz (20&20 s). (a) Shows the U.K./Iceland chain and (b) the Scandinavian
chain.
M.
146
LESTER and
stations on both meridians in H and D components, except for EG and RY which have a dominant component at - 170s. The GEOS-1 data were taken during the time interval 010&0808 U.T., which corresponds to 0431-1039 L.T. The plasma density measurements made by the relaxation sounder experiment on GEOS-1 are shown plotted against L-value and L.T. in Fig. 2. The position of the plasmapause on both the outbound pass, at -0440 L.T., and inbound pass, at - 1030 L.T., can be regarded as being located at an L value of -5.0 (R. Gendrin, personal communication). Chappell et al. (1970) have shown that the plasmapause position within f2 h of 0200 L.T. is determined by the magnetic activity over the preceding 2-6 h. The measurements of plasma density made at 0440 L.T. are very close to this time interval and the observations of Pi 2 pulsations made at -0047 and -0215 L.T. are made during this time interval. In this particular case the average K, in the 6 h immediately before the ground measurements were taken is the same as the average K, in the 6 h before the satellite measurements were made, the average value being 20. We then model the plasmapause position by a line of constant L value DAY
332
D. ORR
at - 5.0 from 0440 L.T. back to - 0047 L.T., although from the average plasmapause position (Carpenter, 1966) there is a difference of AL g 0.5 between these times, the plasmapause being at a higher L shell at the earlier time. Now consider the characteristics of the Pi 2 pulsation over the two meridional chains of stations (Fig. 3). The values of H (geomagnetic North-South) and D (geomagnetic East-West) amplitude represent the maximum values during the event as measured by complex demodulation. The values of H and D phase, ellipticity and ellipse orientation are the values corresponding to the time of the maximum total horizontal wave vector for the event. The ellipticity is inticlockwise looking down the magnetic field line in the Northern Hemisphere for positive values and clockwise for negative values. The ellipse orientation is in the North-East quadrant if positive and in the NorthWest quadrant if negative. The event seen on the U.K. meridian is measured at a central period of 116 s and on the Scandinavian meridian at 111 s ; the difference is due to the spectral resolution. (On both meridians the actual central period lies between 116 s and 111 s, but closer to the periods quoted for each chain.) 1977
GEOS
1
10 -
1
4.8
5.0
1 1028 I II 04310441 0447
I
I
I
6-O
7.0
8-O
FIG.
2.
I 0836
I 0919
I 1039
I 0503
I 0529
I 0550
I 0611
L VALUE I 0754 LT IN I I 0645 0720 LT OUT
ELECTRONDENSITY,~I~~,"s~PROFILEMEASUREDaYTHEGEOS-1RELAXATlONSOUNDEREXPERIMENTON D~~332,1977. The L.T.oftheinbound and outbound passes are also given.
Ground-satellite DAY
332
147
observations of Pi 2 pulsations 1977
Pi2
0046
- 0100 D
H amplitude
UT
amplitude
llf
P
Ellipse H phase
w
68
5f
66
0
64
3
62
D phase
Ellipticity
Orientation
58 56
l
U K meridian
A
ELLIPTICITY
- ve + ve
Scan
meridian
Z clockwise Z anticlockwise
ELLIPSE
-
ve z NW
quadrant
ORIENTATION
+ ve L NE
quadrant
FIG. 3. VALUES OF H AND D AMPLITUDE, H AND D PIIASE AND HORIZONTAL ORIENTATIONCOMPUTEDUSINGCOMPLEXDEMODULATION ATEACHSTATIONINTHETWO FIG. See
ELLIPTICITY AND ELLIPSE CHAINSFGRTHE Pi2 IN
1.
text for details.
Figure 3 shows that the H component amplitude has maxima at RY and LL on the U.K. meridian and at KI on the Scandinavian array. In general, the H component amplitude is stronger on the U.K. meridian than on the Scandinavian meridian. For the D component there are amplitude maxima at RY and KI i.e. one on each meridian, and the amplitudes are similar along both meridians. The secondary H component
maximum observed on the U.K. meridian, in the range 2.94 < L < 3.75, occurs well inside the plasmapause as observed by and modelled from the GEOS-1 plasma density measurements. The single Scandinavian maximum recorded at KI may be related to the plasmapause, and this will be discussed in greater detail in a later section. Turning now to the variation of the H component
M. LESTERand D. ORR
148 EG H
EG D
RY H
FA
H
DU H
LL
DU 0
H
LL
ES H
ES D
CA H
CA D
I
1
22
D
55
23
I
I
10
23
25
23
40
I 22.55
FIG.4(a)
I
I
23
IO
23
I
25
23
IO
Ground-satellite TR
W
5.0
nT
observations
149
of Pi 2 pulsations TR D
:
YI D
KI H T
T 17 0 nT
T
7 0 nT
OL D
OL H
3 0 nT
T 3 0 nT
NU D
NU H
T
I2 OnT
1
I
i
i
22 55
23 10
23 25
23 40
I
I
?2 55
23 I@
I
2325
-II
23 40
F1c.4. Bn~o PASSFILTEREDWAVEFORMSSHOWING 2 Pi2 PULSATIONSOCCURRINGON DAY 333,1977. The filter bandwidth is S-SOmWz(200--20 s) (a) shows the U.K./Iceland chain and(b) the Scandinavian chain.
phase, on the U.K. meridian at L values c 4.46 i.e. for all stations to the South of FA, the H component phase is rather constant with a maximum variation of 23”. EG and RY are out of phase by - 130” and - 155” respectively with respect to FA. On the Scandinavian meridian, there is a gradual change in the H component phase of -90” across L = 5.00, between KI and NU. The D component, however, on the Scandinavian meridian is seen to be closely in phase at all four stations, with a maximum difference of 20”. On the U.K. meridian the D component phase is similar to the H component phase with stations to the South of
FA showing almost constant phase, with a maximum variation of 11”. However, RY lags these stations by - 90” and EG leads them by - 90”. In the variation of ellipticity with latitude, two ellipticity reversals along both meridians are identified. At the U.K. meridian there are reversafs between EG and RY (L = 6.68-L = 6.35) and RY and FA (L = 6.35-L = 4.46) with the latter reversal being from clockwise (RY) to anticlockwise (FA) and the former vice versa. Along the Scandinavian meridian the reversals occur between TR and KI (L = 6.16 L = 5.31) and KI and OL (L = 5.31-L = 4.31). Of
150
M. LESTER and D. ORR
these the latter is from clockwise (KI) to anticlockwise (OL) and is the type of reversal mentioned earlier (Fukunishi, 1975; Lester and Orr, 1981). This reversal occurs at a similar L value to that of the plasmapause and is likely to be related to the plasmapause. The H component phase appears almost constant inside the plasmapause, as does the D component phase. As the plasmapause is crossed, however, the H component phase changes, with the station inside (e.g. NU) lagging the station outside (e.g. KI). There is no change in D component phase across the plasmapause. There is no apparent relationship between the orientation of the major axis of the ellipse and the plasmapause, although there is a longitudinal variation. Only EG on the U.K. meridian does not have the major axis in the NW quadrant, whereas for all the stations in Scandinavia the major axis is in the NE quadrant.
TABLE~.DOMINANTPERIODINTHE H AND D COMPONENTSFOR THET-WOPULSATIONSSHOWNINFIG.~ONDAY~~~,~~~~.NOTE THAT 142.2AND 128.0ARE ADJACENT SPECTRAL ESTIMATESAS ARE 85.3AND 80.0,67.4AND 64.0 AND 160.0 AND 142.2
(b) Day 333 1977 Two Pi 2 pulsations are observed on day 333, at 2302 U.T. and 2322 U.T. (Fig. 4), corresponding to 2302 L.T. and 2322 L.T. on the U.K. meridian and -0030 L.T. and 0050 L.T. on the Scandinavian meridian, note the timing errors at DU and LL. Figure 4(a) shows that both pulsations have similar waveforms from DU to CA on the U.K. meridian, but EG, RY and to a lesser extent FA have more irregular waveforms. Also, the H and D components appear to be different in both events, with the H component having a shorter period in both cases. Comparing the U.K. (Fig. 4a) data with the Scandinavian (Fig. 4b) data, although Pi 2 pulsations occur at the same time, the waveforms are very different on the Scandinavian chain from those observed in the U.K. and Iceland. This is even seen at NU, the lowest latitude station in
present in D. This might suggest that more than one process is present at lower latitudes. Comparing the Scandinavian periods with those from the U.K. there is almost total disagreement in dominant period, unlike the previous event. It should also be noted, however, that the shorter H component dominant period, 8&85 s for event 1 and 71-75 s for event 2, are both present at all the Scandinavian stations, whereas the longer D component dominant period appears only in TR H and D and NU D for event 1 and TR H for event 2. Cold plasma density measurements made on BEE-1 show a well defined plasmapause at L - 4.5 (Fig. 5). The measurements were made in the interval - 203& - 2300 U.T., corresponding to -023@0509 L.T. The relationship between the local time of the Pi 2 pulsations and those of the ISEE- plasma measurements is very similar to the previous event (day 332) and in the same way the plasmapause might be modelled by a line of constant L value from the L.T. of observation, -0330 L.T., to that of the pulsations, 2300-0050 L.T. As only the shorter periods are present in all of the Scandinavian H and D components, complex demodulation results are presented for 83.1 and 74.5 s for events 1 and 2 respectively. For event 1 and T = 83.1 s (Fig. 6) the H amplitude is maximum at EG and KI with a very small secondary maximum at LL. The U.K. amplitude is larger than the Scandinavian. With the plasmapause just poleward of the FA field line, L - 4.5, it is unlikely that the secondary maximum at LL is associated with this feature directly, nor indeed is the maximum at EG. This is associated with the field line on which the Pi 2 pulsation is generated. The D amplitude shows maxima at RY and KI. Comparing H with D, only at OL and NU is D the strongest component and it is
Scandinavia,
which in general might be expected
have similar waveforms
to those
to
seen at LL and ES (Meir-Jedrzejowicz and Southwood, 1979). The differences in the waveforms are highlighted by the power spectra for each event. If the dominant spectral component is considered, Table 2, from FA to CA for event 1 (230&2320 U.T.) the dominant period is the same in the H and D components, except for FA D and CA H. EG tends to show some agreement but RY has a much longer dominant period especially with H component. For event 2, FA to CA again show the same dominant period, but in this case RY agrees with the lower latitude stations and EG differs. Another important feature is that the H component has a shorter dominant period than the D component for both events. The longer period is present in H in both events although at a shorter period in event 2 (128 s as opposed to 142 or 160 s), and the shorter period is
Time interval Station
EG RY FA DU LL ES CA TR KI OL NU
2300-2320 U.T. Period (s) H D
232lF2340 U.T. Period (s) H D
91.4 142.2 80.0 85.3 85.3 85.3 67.4 64.0 67.4 53.3 80.0
160.0 75.3 75.3 75.3 75.3 75.3 75.3 128.0 182.9 49.2 41.3
128.0 160.0 55.7 116.4 128.0 116.4 128.0 106.7 85.3 85.3 64.0
75.3 160.0 160.0 160.0 142.2 160.0 142.2 49.2 106.7 71.1 71.1
Ground-satellite
1000
151
observations of Pi 2 pulsations
-
100 0 ‘E ”
,” C
lo-
l-
.l 2
I
I
I
I
I
I
3
4
5
6
7
0
20.45
23.00
UT
03.00
05.09
LT
MEASURED BY THEISEEFIG. 5. ELECTRONDENSITYvs L PROFILE 333.1977. DAY
333
1977
RELAXATION SOUNDER EXPERIMENT ON DAY
Ev.1
fi amplitude
D amplitude
66
58
[ -i
L
:lIo-_ 6
8
10
12
1
L______ 0
2
4
6
6
10
IiT D phase
-180
12
nT
0
Ellipse Orientation
Ellipticity
+lEo
-1
0
FIG. 6. As FORFIG. 3 FOREVENT1 ON DAY
+1
-90
0
333, 1977.
+90
152
LPSTER and
M.
only at these two stations that the Scandinavian amplitude is larger than the U.K. amplitude. Because of timing errors phase measurements at DU and LL are not included. In the H component phase ES lags CA by - 20” and FA lags ES by 75”. A further difference of -50” in the same direction occurs between FA and RY, but EG leads RY by -90”. On the Scandinavian chain TR and KI are “in phase” but OL differs by -50” and the largest phase difference occurs between NU and OL. It seems that the large phase differences occur across the plasmapause. In the D component phase there is only a similar large phase difference between EG and RY i.e. no phase difference across the plasmapause. The ellipticity shows that there is a change in the sense of polarisation between FA and DU on the U.K. meridian and OL and NU on the Scandinavian meridian. Poleward of this reversal all stations exhibit clockwise rotation and equatorward anticlockwise rotation. This is again the reversal referred to by Fukunishi (1975), Lester and Orr (1981) and others.
DAY
The reversal, however, does occur at a slightly lower L shell than the plasmapause. The ellipse orientation shows similar characteristics to the event on day 332 i.e. only EG on the U.K. meridian has the major axis of polarisation in the NE quadrant. Note that on the Scandinavian chain TR and KI exhibit an almost North-South orientation, but OL and NU show an almost East-West orientation. This confirms the dominance of H at the former two stations and of D at the latter. In discussing the characteristics of the second event (Fig. 7), we shall compare them with those for event 1. In the H amplitude the maxima again occur at EG, KI and LL, a small secondary maximum. In D however the maximum has moved from RY to EG. Also again only OL and NU have D as the dominant magnetic component. In the phase characteristics, there is now a larger difference between FA and RY and a smaller one between FA and ES in the H component. In the D component the phase again remains almost constant across
1977
333
D. Oaa
Ev. 2 D amplitude
H amplitude
62
2 J
0 F w z
501. 0
' 2
' 4
' 6
' 8
' 10
' 12
J 14
IfI 0
1
4
6
6
10
flT H phase
D phase
IlT Ellipticity
Ellipse Orientation
FIG. 7. As FOR FIG. 3 FOR EVENT2 ON DAY 333, 1971.
Ground-satellite
153
observations of Pi 2 pulsations
the plasmapause, but TR differs from KI by - 100”. The ellipticity reversal on the U.K. meridian has moved North of FA but stayed between OL and NU on the Scandinavian chain. This is obviously related to the difference in H component phase seen between events 1 and 2. The orientation of the major axis has
changed, with FA to CA all oriented almost directly North-South, i.e. H is much larger than D, and RY in the NE quadrant and EG in the NW. On the Scandinavian chain TR and KI are now in the NW quadrant, but OL and NU remain almost East-West in direction.
D camp
H corn*
I
4 0 nT
LE -
1
23
,
40
23
I
55
0 IO
I
025
I
043
I
055
J
I
I
1
I 10
23 40
23 55
0 13
FE. 8(a)
1
025
,
I
0 40
0 55
I
IO
154
iI&. LESTERand M
r).
ORR
camp
D camp
I
I
l&OPT
This particular interval is interesting because 3 Pi 2 pulsation events occur close to midnight on day 118, The start times are at 2355 U.T. on day I 18,0030 U.T. and 0045 U.T. on day 119 and the events wiu be known 88 events I, 2 and 3 respective@. Aft three events are seen along both meridians (Fig. 8) although there are again differences between the two meridians e.g. events 2 and 3 are considerably stronger in the D component on Ehe Scandinavian chain. Each. PI 2 pulsation consisted d more than one spectral eomponen$
I
12 0 nT
and considerable variation in the daminant pepetiwrd especially for event 2. For event X the dominant spectral component on the U.K./lcelnnd meridian was at -71 s except at EG where it was at - 150 s. On the ~ca~~a~~a~ meridian the don~~nan~ spectrai ~om~onen~w~ at - 71 s in B, but in ff at TR and KL it was -iIisaadatQLandNUY71s,ForeventZjllD the dominant spectral component ~88 at - 90 s except at EG where it was at - 150 s. For the R component: it varies considerably with EG and KS at - 150 s, Of;, NU_ FA and LE at - 90 s, LL at w 76 s and ES, Y@
Ground-satellite
155
observations of Pi 2 pulsations
and CA at - 55s.The D component was the strongest of the two horizontal components and so the spectral component at -90 s was chosen in the further complex demodulation analysis. For event 3, the dominant component was at -68 s at all stations except EG where it was at - 150 s. Again ISEEcold plasma density measurements (Fig. 9) have been used to estimate the plasmapause position. In this case, however, no unambiguous determination can be found, although a change in plasma density gradient does occur at L - 3.7. The plasma measurements and the Pi 2 pulsations occur very close in L.T., 0144-0403 L.T. for the former and 235550215 L.T. for the latter, but the plasma measurements were made some 8 h in U.T. after the occurrence of the pulsations. The observations were again made in the sector where Chappell et a/. (1970) found the plasmapause position to be determined by the magnetic activity over the preceding 226 h. For both sets of observations this is very similar; K, averages 20 for the 6 h preceding the occurrence of the Pi 2 pulsations and 1+ preceding the plasma density measurements. We suggest that the profile in Fig. 9 is, therefore, similar to the one which was present at the time of the pulsations’ occurrence.
From the results presented in the first two parts of this section it appears that two parameters of the Pi 2 pulsations are sensitive to the position of the plasmapause-El component phase and ellipticity. For the three events in this time interval, therefore, we present only the latitudinal variations of these parameters (Fig. 10). (Note that due to a timing error H component phase information from ES will not be presented.) If we consider first the U.K. H component phase variation, one can see that the difference in phase between FA and LE increases from event to event, while the phase variation from LE to CA is reasonably constant. The phase difference between FA and LE increases from 25” for event 1, to 45” for event 2, to 117” for event 3. Comparing this variation with those seen for the previous 2 events, it is suggested that the plasmapause may be moving radially inwards. The Scandinavian variation in H component phase suggests something similar happening in this meridian, with large variations between KI and OL for events 1 and 3, 82 and 90” respectively, and a rather more gradual change for event 2, from KI to NU, of 77”. The variation of ellipticity with latitude on the U.K. meridian shows a reversal between EC and FA for
0144
0403
FIG.~. ELECTRONDENSINVSLPROFILEMEASIJREDBYTHE
BEE-1
119, 1978.
LT RELAXATIONSOUNDEREXPERIMENTON
DAY
156
M.
LESTERand D. ORR
DAY
118/g
H phase Ev. 2
Ev.1
Ev.3
62
:
54
1 IF a
50 -180
0
f180
Ellipticity
I
58-
54-
L
”,lL !k I
I
50 -1.0
“1 d 0
a.
+1.0
0
-1.0
+ 1.0
-1.0
0
t1.0
FIG.10. VALUES OF H PHASE AND HORIZONTAL ELLIPTICITY FORTHETHREEPi2 PULSATIONS ON DAY 118/9,
1978.
event 1, and between FA and LE for event 3. This is again consistent with a radially inward motion of the plasmapause. The lack of a reversal for event 2 may be explained if the Pi 2 is generated to the South of EG. In this case a reversal in ellipticity would occur at this position with a negative sense of rotation to the South of the generation region (Saito et al., 1976). This in turn would suggest that there is another reversal to the North of FA, which is the one identified with the plasmapause. It is, however, difficult to rationalise the latitudinal variation in ellipticity on the Scandinavian chain for events 1 and 2 in this way and this will be discussed further in the next section. For event 3 there is a reversal between KI and OL. Comparing the above results with the plasma density profile, it seems only event 3 on the U.K. meridian has a reversal which is close to the change in gradient at L - 3.7. The fact that events 1 and 2 do not show this is not unreasonable as some radial motion of
the plasmapause is expected in such magnetic conditions. The difference in position of the reversal between the two meridians may be due to differing positions of the plasmapause and/or plasma gradients. 3. DISCUSSION
The first two comparisons between Pi 2 pulsation characteristics and plasma density measurements, days 332 and 333 1977, suggest that only H component phase and the horizontal ellipticity vary across the plasmapause. The secondary H component amplitude maximum (Stuart and Baransky, 1983) appears to occur well inside the plasmapause. It should be noted that this feature is not well defined for the two pulsations on day 333 or for the three events on days 118/9. On the Scandinavian chain on day 332 there appears a maximum at KI (I_.= 5.31) which is very close to the plasmapause position (L - 5.0).
Ground-.satetiite II has been suggested
observations
(Rostoker and Samson, 1981) the peak power of the Pi 2 pulsation occurs at the equatorward edge of the westward eiectrojet for morning sector events and along the Harang discontinuity for evening sector pulsations. On day 332 a westward eiectrojet is present over both EG and RY and TR and KI at 0045 U.T., just before the Pi 2 onset. The centre of this electrojet is poleward of EG and very close to KI, and the equatorward edge can only be determined as being South of EG (but well to the North of FA) and South of KI (and again well to the North of OL). It seems then that the maxima on both meridians are associated with the electrojet system, although the maximum of Kl may also be associated with the plasmapause. Since it is generally believed that the main source of Pi 2 pulsations is in the electrojet system (Rostoker and Samson, 1981; Samson, 198 1; Pashin et al., 1983fitis more likely to be the former. The two events on day 333 have similar electrojet systems at the time of Pi 2 onset. A westward electrojet is present over Iceland and Scandinavia for both events, the only difference being the movement of the centre of the electrojct North of EG for event 2. This may imply that the equatorward edge is closer to RY but still remaining South of RY. Over Scandinavia the centre is to the North ofKI and the equatorward edge to the South. The electrojet is much stronger, by a factor of -3, during the second event. Qn days 1 IS/9 there is an eastward eiectrojet over Scandinavia during the first event, but over Iceland a westward electrojet is abserved at EG (no horizontal magnetic data are present from RY). For the other two events a westward electrojet is seen, with the centre to the North ofEG and equatorward edge North of RY in both cases. There is no indication that the equatorward edge has moved close to and just South of EG as mentioned earl& in discussing the ellipticity properties for event 2, although this cannot be ruled out. Having discussed the effect of the electrojet system on the amplitude of the Pi 2 pulsation, it should also be pointed out that the electrojets and associated field aligned currents may affect the H component phase (J. 6. Samson, pers. comm., 1982) and the sense of polarisation (Samson, 1981). Indeed Samson (1981) contends that the reversal in the sense of polarisation seen at mid-latitudes can be accounted for in a model of the generation of Pi 2 pulsations through a threedimensional current system induding the electrojet systems, and upward field aligned currents which link through an aurora1 arc in the breakup region and downward field aligned currents at the poleward edge of the electrojet border. This may in part account for
that
of A 2 pulsations
157
the difference in the ellipticity between the two meridians for events 1 and 2 on days ll8/9. Other factors may be influential in the position of polarisation reversal at mid-latitudes. For example three mechanisms have been proposed which would modify the Pi 2 signal away from the aurora1 electrojet; plasmaspheric cavity resonance (Doobov and Mainstone, 1973), surface wave on the plasmapause (Southwood and Stuart, 1979) and line resonance (Stuart et at., i979). With thecoincident plasma density measurements one may be able to decide which of these three mechanisms is the most likely candidate. Figure 11 shows the power amphtude spectra in the W component in the frequency range 5-20 mHz (206 50 s) from the time interval 004~-0105 U.T. on day 332, 1977. Three stations are presented, one from the amoral zone which is close to the generation region of the Pi 2, RY; one from near the plasmapause, FA ; and me which is iocated inside the plasmapause, ES. The dominant period at RY is at 174 s, and the half-power points of this spectral peak are marked on Fig. 10 by the lines A and B. This period is not apparent at FA and although there is some power at ES there is no obvious peak. The dominant period at FA is at - I20 s, and at ES it is at N I12 s. Consider first the spectral peak at - 120 s at FA, which has half-power points indicated by lines C and D (Fig. 11). This spectral peak shows up clearly at ES, and there is also a peak at RY although this is not well defined. Finally let us consider the spectral peak at RY at -78 s, with half-power points E and F (Fig. 11).At FA this spectral peak is not seen but at ES there is a well defined peak. The power spectra at ES is typical of that at the other plasmaspheric stations LE, EL and CA. At RY the power is mainly at Longer periods than those seen to dominate at FA and ES. This suggests that the power in the Pi 2 is being modified at the latitudes corresponding to these stations. Using plasma frequency measurements obtained by the GEOS S-300 experimenters, and sent to us by courtesy of T3r. R. Gendrin, the eingenperiods of the guided poloidal wave, T,, were evaluated at four positions using the eigenvalues computed by Orr and Matthew (I 971) (Table 3). The characteristic periods have been obtained by assuming charge neutrality and that the ions are protons. Also a power law distribution of the form n, = n,fr/r$” where n0 and n, are the proton number of densities at r0 and r respectively and m = 3 has been assumed. Chen and Hasegawa (1974) consider the characteristic period of a surface wave that may be generated at a plasma density discontinuity such as the plasmapause. The surface wave will travel over the boundary at a speed which is some average of the Alfven velocity
M. LESTERand D. ORR
158
H Comp.
0.11
5
FIG. 11. H
COMPONENT
I
I
I
10
15
20
mHz
POWERSPECTRA FORTHREESTATIONS FROMTHETIMEINTERVAL 004&0105 U.T. DAY 332,1977, IN THEFREQUENCY RANGE5-20 mHz (200-50 s).
See text for full details.
on the two sides of the boundary. The period of the surface wave, T,, will be given approximately by the longest period within the plasmapause in the guided poloidal mode divided by fi (A. Hasegawa, personal communication). From Table 2 this is seen to be about 109 s, in encouraging agreement with the observed dominant period at the plasmapause and within the plasmasphere. This together with the obvious change in spectra
between RY and FA suggest that the surface wave is dominating at FA and although attenuated inside the plasmapause is the dominant component. Also the component at -78 s, seen at RY and also at ES is swamped by this surface wave at FA. The fact, however, that RY and ES see similar periods suggest that this period is not necessarily related to a surface wave and at ES may simply be the response of “midlatitude” plasma to a forcing source field at higher
Ground-satellite
159
observations of Pi 2 pulsations
TABLE 3. CALCULATED CHARACTERISTIC PERIODS, TI (s)USING THE RESULTS OF ORR MATTHEW(~~~~)ANDTHECHARACTERIS~CSURFACEWAVE,T,(S),USINGTHEELECTRONDENSI~ MEASUREMENTSIN FIG. 2
Position
U.T.
L.T.
L
R/R,
f, &Hz)
n, (cm- 3,
Tl (s)
1
0100
2 3 4
0105 0110 0130
0431 0441 0447 0503
4.98 5.12 5.26 5.71
4.70 4.83 4.95 5.34
60 55 35 20
45 38 15 5
150 154 107 86
latitudes. No other evidence to support this, however has been found from the complex demodulation analysis. Making the same assumptions as before, the characteristic periods have been obtained for the plasma density profile for day 333(Table 4). The surface wave period here is - 170 s. This period is not seen at any of the stations inside the plasmasphere and a comparable period is only seen at RY in the D component at 160 s. It is interesting to note that the dominant H component periods for both events 1 and 2 are close to the second harmonic of the characteristic surface wave. The other difference for the events on day 333 is the apparent decoupling of H and D at stations within the plasmasphere. This suggests that there may have been more than one type of mechanism present for these events. The characteristic periods for the guided poloidal mode based on the plasma density measurements made on day 119 (Table 5) show that there is a decrease in the characteristic period at L - 3.7 which is the position where there is a change in the gradient of the plasma density (Fig. 9). This change in gradient may be
TABLE 4. As FOR TABLE 3 BUT USING THE ELEcTRONDENSlTY MEASUREMENTSIN Fic.5
n, (cm - “)
L
T (4
1000
3.29 4.00 4.25 4.42 4.54 4.65
135 221 237 219 167 58
-
510 348 215 100 10
T (4
167
TABLE 5. As FOR TABLE 3 BUT USING THE ELECTRONDENSITYMEASUREMENTSIN FIG.9 n, (cm 1000
350 190 100
“)
L
TI 6)
2.7 3.6 3.7 4.0
58 114 100 98
x (4
81 _
AND
T, (s) 109
able to support a surface wave which would have the characteristic period of - 81 s. This is in encouraging agreement with the dominant periods of the three events which are in the range 68-93 s, despite the variation in spectral content especially in event 2.
4.CONCLUSlONS
We have attempted to correlate satellite plasma density measurements with the latitudinal variations of the amplitude, phase and polarisation parameters of Pi 2 pulsations measured on two meridional chains of magnetometers separated by - 1s h in time. Several conclusions can be made : (a) There appears to be a close relationship between the position of the plasmapause and the position of an ellipticity reversal. (b) There is also a variation in H component phase across the plasmapause position which is not seen in the D component phase. (c) Characteristic surface wave periods calculated using the cold plasma density measurements appear to be present in the spectra of stations inside the plasmapause for four of the six Pi 2 events. (d) At times there is some decoupling of H and D so that each magnetic component will have a different dominant period. (e) On occasions there is no high degree of coherency between stations inside the plasmapause. Of the three mechanisms mentioned earlier, the surface wave on the plasmapause appears to play the dominant role at mid-latitudes. A cavity resonance would typically give periods in the range 4&50 s for the plasma measurements presented here and there is not a sufficiently strong secondary amplitude maximum for the line resonance to be considered. Also for a line resonance an ellipticity reversal should occur close to the secondary maximum which is not the case. Events do occur, such as those on Day 333‘where the calculated surface wave periods do not agree with the observed dominant periods and such cases therefore may not be due to the surface wave mechanism. The important feature in this case is the lack of coherency
160
M. LESTER and
between stations in the U.K. and those in Scandinavia which may be a means of identifying such cases when plasma density measurements are unavailable. It is also possible that factors other than the plasmapause play an important role at mid-latitudes (Samson, 1981). This would certainly be true close to the meridian of the source of the Pi 2 pulsations and during extremely disturbed conditions and strong electrojets. The role of the electrojets and field aligned currents need to be modelled further and the plasmapause needs to be taken into account in the model. Also further experimental work with a closely spaced magnetometer array, covering both mid-latitudes and the aurora1 regions with coincident satellite measurements of the plasma density needs to be carried out.
Acknowledgements-We would like lo acknowledge the help of all the people who have operated magnetometer stations since the network was set up ; in particular the observers at the 13 sites used in this paper. We would also like to acknowledge the work of Dr W. F. Stuart who co-ordinated the network. We thank Drs R. Gendrin and J. Etcheto for supplying the plasma density measurements. Thanks go to Drs M. R. Warner and C. A. Green for helpful discussions. One of us (M.L.) was supported by the SERC on research grant SD 02462.
REFERENCES Beamish, D., Hanson, H. W. and Webb, D. C. (1979). Complex demodulation applied to Pi 2 geomagnetic pulsations. Geophys. J. R. astr. Sot. 58,471. Carpenter, D. L. (1966). Whistler studies of the plasmapause in the magnetosphere. 1. Temporal variations in the position of the knee and some evidence on plasma motions near the knee. J. geophys. Res. 71,693. Chaonell. C. R.. Harris. K. K. and SharD, G. W. (1970). A study of the influence bf magnetic activiiy on the l&at&n of the plasmapause as measured by OGO 5. J. geophys. Res. 75, 50. Chen, L. and Hasegawa, A. (1974). A theory of long period magnetic pulsations. 2. Impulse and excitation of surface eigenmodes. J. geophys. Res. 79, 1032. Doobov, A. L. and Mainstone, J. S. (1973). Investigations of
D. ORR
Pi 2 micropulsationsRelevance of observations generation theories. Planet.Space Sci. 21,731.
to
Etcheto, J. (1976). Diagnosis of spatial plasmas with a relaxation sounder, in The Scientific Satellite Programme During the International Magnetospheric Study. (Edited by Knott, K. and Battrick, J.), p. 87. D. Reidel, Dordrecht. Fukunishi, H. (1975). Polarisation changes of geomagnetic Pi 2 pulsations associated with the plasmapause. J. geophys. Res. 80, 98. Higel, B. (1978). Small scale structure of magnetospheric electron density through on-line tracking of plasma resonances. Space Sci. Rev. 22, 6 11. Lester, M. and Orr, D. (1981). The spatio-temporal characteristics of Pi 2’s. J. almos. terr. Phys. 43, 947. Mier-Jedrzejowicz, W. A. C. and Southwood, D. J. (1979). The East-West structure of mid-latitude geomagnetic pulsation in the 8&25 mHz band. Planet. Space Sci. 27, 617. Orr, D. and Matthew, J. A. D. (1971). The variation of geomagnetic micropulsation periods with latitude and the plasmapause. Planet. Space Sci. 19, 897. Orr, D. and Webb, D. C. (1975). Statistical studies of geomagnetic pulsations with periods between 10 and 70 s and their relationship to the plasmapause region. Planet Space Sci. 23, 1169. Pashin, A. B., Glassmeier, K. H., Baumjohann, W., Raspopov, 0. M., Yahnin, A. G., Opgenoorth, H. J. and Pellinen, R. J. (1983). Pi 2 magnetic pulsations, aurora1 break-ups, and the substorm current wedge: a case study. J. geophys. Res. (in press). Rosloker, G. and Samson, J. C. (1981). Polarization characteristics of Pi 2 pulsations and implications for their source mechanisms : location of source regions with respect to the aurora1 electrojets. Planet. Space Sci. 29, 225. Saito, T., Sakurai, T. and Koyama, Y. (1976). Mechanisms of association between Pi 2 pulsation and magnetospheric substorms. J. atmos. terr. Phys. 38, 1265. Samson, J. C. (1981). Pi 2 pulsations: High latitude results. Paper presented at 41h IAGA Assembly, Edinburgh, August 1981. Southwood, D. J. and Stuart, W. F. (1979). Pulsations at the substorm onset, in Dynamics qfthe Magnetosphere. (Edited by Akasofu, S.-I.), p. 341. D. Reidel, Dordrecht. Stuart, W. F. and Baransky, L. (1983). Simultaneous observations of Pi 2 pulsations on N-S meridians in the U.K. and Scandinavia. (Submitted to J. atmos terr. Phys.) Stuart, W. F., Brett, P. M. and Harris, T. J. (1979). Midlatitude secondary resonance in Pi 2’s. J. atmos. terr. Phys. 41, 65. Webb, D. C. (1979). The analysis ofnon-stationary data using complex demodulation. Ann. Telecommun. 35, 131.
Phet.SpoceScr., Vol. 31,No.Z,pp.143~160,1983 Printed m Great Britain.
CORRELATIONS BETWEEN GROUND OBSERVATIONS OF Pi 2 GEOMAGNETIC PULSATIONS AND SATELLITE PLASMA DENSITY OBSERVATIONS* Department
M. LESTER? and D. ORR of Physics, University of York, Heslington, (Received
infinulform
York, YOl SDD, U.K.
5 July 1982)
Abstract -Ground observations of Pi 2 geomagnetic pulsations are correlated with satellite measurements of plasma density for three time intervals. The pulsations were recorded using the IGS network of magnetometer stations and the plasma density measurements were made on board GEOS-1 and ISEE-1. Using the technique of complex demodulation, the amplitude, phase and polarisation characteristics of the Pi 2 pulsations are observed along two meridional profiles ; one from Eidar, Iceland (L = 6.7) to Cambridge, U.K. (L = 2.5) and the other from Tromso, Norway (L = 6.2) to Nurmijarvi, Finland (L = 3.3). The observed characteristics of the Pi 2 pulsations are then compared with the plasma density measurements. Close relationships between the plasmapause position and the position of an elliptic&y reversal and a variation in H component phase are observed. A small, secondary amplitude maximum is observed on the U.K./Iceland meridian well inside the position of the projection of the equatorial plasmapause. The primary maxima on the two meridians, in general occur close to the estimated position of the equatorward edge of a westward electrojet. Using the plasma density measurements, the periods of surface waves at the plasmapause for two intervals are estimated and found to be in good agreement with the dominant spectral peaks observed at the ground stations near the plasmapause latitude and within the plasmasphere. The polarisation reversal, together with phase characteristics, spectral evidence and the agreement between the theoretical and observed periods leads to the suggestion that on occasions a surface wave is excited on the plasmapause as an intermediate stage in the propagation of Pi 2 pulsations from the aurora1 zone to lower latitudes. 1. INTRODUCTION
(Carpenter, 1966), (see Fig. 12 of Lester and Orr (1981)). This type of study has been extended (Stuart and Baransky, 1982) by using two meridional profiles, one in the U.K. and one in Scandinavia. Stuart and Baransky (1982) used a statistically derived position for the plasmapause based on the results of Chappell et al. (1970) and Orr and Webb (1975). Using a very simple analysis method, Stuart and Baransky (1982) inferred that the sense of polarisation and the spectral structure of the Pi 2 changed at the plasmapause, but also that a minimum in the H component amplitude also occurred there. Stuart and Baransky (1982) suggested that Pi 2 pulsations excite a cavity resonance in the plasmasphere. Using an average or a statistically derived plasmapause position does not describe accurately the relationship between the variations of Pi 2 characteristics, in particular the polarisation reversal, and the plasmapause because of the variability of the plasmapause position which is related to substorm activity. In an attempt to obtain a more precise relationship between these two features, we have studied occasions when satellite data are available simultaneously with the ground magnetic signature of Pi 2 pulsations. In particular measurements of plasma density in the near magnetosphere (2 < L < 7) are correlated with the characteristics of Pi 2 pulsations observed on two meridional chains of magnetometers
The latitudinal characteristics of Pi 2 geomagnetic pulsations have been extensively studied using arrays of ground based magnetometers at middle latitudes (e.g. Fukunishi, 1975 ; Stuart et al., 1979; Lester and Orr, 1981) and in the aurora1 zone (Rostoker and Samson, 1981). The main purposes of the mid-latitude studies have been to determine the mode of propagation of Pi 2 pulsations at these latitudes and to identify any characteristics which may be used as diagnostics of the plasmapause. In a recent study, Lester and Orr (1981) showed statistically that there is a reversal in the sense of polarisation at middle latitudes (corresponding typically to L values in the range 3.2-4.5) from clockwise, looking down the field line in the Northern Hemisphere, to anticlockwise as one traverses this middle latitude region from high to low latitudes. This confirmed the earlier work of Fukunishi (1975). The position of the L value of the reversal was observed to have a close correlation with the average position of the plasmapause deduced from whistler observations
*Paper presented at the Fourth IAGA Assembly, Edinburgh, 3-15 August 1981. t Present address : Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, U.S.A. 143
M. LESTERand D.
144
ORR
TABLE1. STATIONS USED
IN THIS PAPER. Atso SHOWN ARE THE SYMBOL, ANDTHE~VALUEANDCORRECTEZDMAGNETICLATITUDEANDLONGITUDE VALUES AT THE EARTH'SSURFACE FOR IGRF(FIRST 7 HARMONICS) 1975
(COMPUTED BYM. R. WARNER) Station
28 0
-
EG RV FA LE DU LL ES YO CA
67.2 66.6 61.7 58.9 58.0 56.2 54.3 52.6 50.6
74.5 71.2 81.3 84.3 80.9 80.4 80.5 81.6 81.8
6.68 6.35 4.46 3.75 3.56 3.23 2.94 2.71 2.48
Tromso Kiruna Oulu Nurmijarvi
TR KI CL NU
66.2 64.3 61.2 56.6
105.4 104.8 106.7 103.6
6.16 5.31 4.31 3.31
EG
H
20 0
component I
I
1
I
28.0
40 0
12 0
I2 0
1 1
1
80
80 I
I T
40
80
1
I
40
80
I
I
40
80
1
I
0.40
L value
Eidar Leirvogour Torshavn Lerwick Durness Loch Laggen Eskdalemuir York Cambridge
nT
i
Corrected magnetic E Longitude Latitude
Symbol
0 45
0 50
0 55
10
FIG. l(a)
nT
Ground-satellite
during three intervals. The plasma density data are taken from the relaxation sounder experiments on board GEOS-1 (Higel, 1978) and ISEE(Etcheto, 1976). The magnetometer data are taken from the IGS/Imperial College/University of York network of stations in the U.K., Iceland and Scandinavia (Table 1). The position of the plasmapause is compared with the latitudinal variations of, not only the polarisation parameters, but also amplitude and phase obtained using the technique of complex demodulation (Beamish et al., 1979; Webb, 1979) and spectral characteristics. TR day 332
145
observations of Pi 2 pulsations 2. THE DATA
(a) Day 332 1977 A Pi 2 pulsation was observed at 0047 U.T. along both the U.K./Iceland and Scandinavian meridional chains (Fig. 1). This is equivalent to - 0047 L.T. for the U.K. meridian and -0215 L.T. for the Scandinavian meridian. From Fig. 1 it can be seen that there appear to be two separate parts to the event, with different periods. Power spectral analysis of these two parts shows that in the first part the dominant period is -70s and in the second - 120s. This is true at all
1977 H component
TR
day 332
1977
D component
T
1
5 OnT
2 OnT
I
I
60
60
I
NU
NU
I 0
i
F1c.1. BAND PASSFILTEKED WAVEFORMS SHOWING A Pi2 PULSATIONOCCURRING ON DAY 332,1977. The filter bandwidth is 5-50 mHz (20&20 s). (a) Shows the U.K./Iceland chain and (b) the Scandinavian
chain.
M.
146
LESTER and
stations on both meridians in H and D components, except for EG and RY which have a dominant component at - 170s. The GEOS-1 data were taken during the time interval 010&0808 U.T., which corresponds to 0431-1039 L.T. The plasma density measurements made by the relaxation sounder experiment on GEOS-1 are shown plotted against L-value and L.T. in Fig. 2. The position of the plasmapause on both the outbound pass, at -0440 L.T., and inbound pass, at - 1030 L.T., can be regarded as being located at an L value of -5.0 (R. Gendrin, personal communication). Chappell et al. (1970) have shown that the plasmapause position within f2 h of 0200 L.T. is determined by the magnetic activity over the preceding 2-6 h. The measurements of plasma density made at 0440 L.T. are very close to this time interval and the observations of Pi 2 pulsations made at -0047 and -0215 L.T. are made during this time interval. In this particular case the average K, in the 6 h immediately before the ground measurements were taken is the same as the average K, in the 6 h before the satellite measurements were made, the average value being 20. We then model the plasmapause position by a line of constant L value DAY
332
D. ORR
at - 5.0 from 0440 L.T. back to - 0047 L.T., although from the average plasmapause position (Carpenter, 1966) there is a difference of AL g 0.5 between these times, the plasmapause being at a higher L shell at the earlier time. Now consider the characteristics of the Pi 2 pulsation over the two meridional chains of stations (Fig. 3). The values of H (geomagnetic North-South) and D (geomagnetic East-West) amplitude represent the maximum values during the event as measured by complex demodulation. The values of H and D phase, ellipticity and ellipse orientation are the values corresponding to the time of the maximum total horizontal wave vector for the event. The ellipticity is inticlockwise looking down the magnetic field line in the Northern Hemisphere for positive values and clockwise for negative values. The ellipse orientation is in the North-East quadrant if positive and in the NorthWest quadrant if negative. The event seen on the U.K. meridian is measured at a central period of 116 s and on the Scandinavian meridian at 111 s ; the difference is due to the spectral resolution. (On both meridians the actual central period lies between 116 s and 111 s, but closer to the periods quoted for each chain.) 1977
GEOS
1
10 -
1
4.8
5.0
1 1028 I II 04310441 0447
I
I
I
6-O
7.0
8-O
FIG.
2.
I 0836
I 0919
I 1039
I 0503
I 0529
I 0550
I 0611
L VALUE I 0754 LT IN I I 0645 0720 LT OUT
ELECTRONDENSITY,~I~~,"s~PROFILEMEASUREDaYTHEGEOS-1RELAXATlONSOUNDEREXPERIMENTON D~~332,1977. The L.T.oftheinbound and outbound passes are also given.
Ground-satellite DAY
332
147
observations of Pi 2 pulsations 1977
Pi2
0046
- 0100 D
H amplitude
UT
amplitude
llf
P
Ellipse H phase
w
68
5f
66
0
64
3
62
D phase
Ellipticity
Orientation
58 56
l
U K meridian
A
ELLIPTICITY
- ve + ve
Scan
meridian
Z clockwise Z anticlockwise
ELLIPSE
-
ve z NW
quadrant
ORIENTATION
+ ve L NE
quadrant
FIG. 3. VALUES OF H AND D AMPLITUDE, H AND D PIIASE AND HORIZONTAL ORIENTATIONCOMPUTEDUSINGCOMPLEXDEMODULATION ATEACHSTATIONINTHETWO FIG. See
ELLIPTICITY AND ELLIPSE CHAINSFGRTHE Pi2 IN
1.
text for details.
Figure 3 shows that the H component amplitude has maxima at RY and LL on the U.K. meridian and at KI on the Scandinavian array. In general, the H component amplitude is stronger on the U.K. meridian than on the Scandinavian meridian. For the D component there are amplitude maxima at RY and KI i.e. one on each meridian, and the amplitudes are similar along both meridians. The secondary H component
maximum observed on the U.K. meridian, in the range 2.94 < L < 3.75, occurs well inside the plasmapause as observed by and modelled from the GEOS-1 plasma density measurements. The single Scandinavian maximum recorded at KI may be related to the plasmapause, and this will be discussed in greater detail in a later section. Turning now to the variation of the H component
M. LESTERand D. ORR
148 EG H
EG D
RY H
FA
H
DU H
LL
DU 0
H
LL
ES H
ES D
CA H
CA D
I
1
22
D
55
23
I
I
10
23
25
23
40
I 22.55
FIG.4(a)
I
I
23
IO
23
I
25
23
IO
Ground-satellite TR
W
5.0
nT
observations
149
of Pi 2 pulsations TR D
:
YI D
KI H T
T 17 0 nT
T
7 0 nT
OL D
OL H
3 0 nT
T 3 0 nT
NU D
NU H
T
I2 OnT
1
I
i
i
22 55
23 10
23 25
23 40
I
I
?2 55
23 I@
I
2325
-II
23 40
F1c.4. Bn~o PASSFILTEREDWAVEFORMSSHOWING 2 Pi2 PULSATIONSOCCURRINGON DAY 333,1977. The filter bandwidth is S-SOmWz(200--20 s) (a) shows the U.K./Iceland chain and(b) the Scandinavian chain.
phase, on the U.K. meridian at L values c 4.46 i.e. for all stations to the South of FA, the H component phase is rather constant with a maximum variation of 23”. EG and RY are out of phase by - 130” and - 155” respectively with respect to FA. On the Scandinavian meridian, there is a gradual change in the H component phase of -90” across L = 5.00, between KI and NU. The D component, however, on the Scandinavian meridian is seen to be closely in phase at all four stations, with a maximum difference of 20”. On the U.K. meridian the D component phase is similar to the H component phase with stations to the South of
FA showing almost constant phase, with a maximum variation of 11”. However, RY lags these stations by - 90” and EG leads them by - 90”. In the variation of ellipticity with latitude, two ellipticity reversals along both meridians are identified. At the U.K. meridian there are reversafs between EG and RY (L = 6.68-L = 6.35) and RY and FA (L = 6.35-L = 4.46) with the latter reversal being from clockwise (RY) to anticlockwise (FA) and the former vice versa. Along the Scandinavian meridian the reversals occur between TR and KI (L = 6.16 L = 5.31) and KI and OL (L = 5.31-L = 4.31). Of
150
M. LESTER and D. ORR
these the latter is from clockwise (KI) to anticlockwise (OL) and is the type of reversal mentioned earlier (Fukunishi, 1975; Lester and Orr, 1981). This reversal occurs at a similar L value to that of the plasmapause and is likely to be related to the plasmapause. The H component phase appears almost constant inside the plasmapause, as does the D component phase. As the plasmapause is crossed, however, the H component phase changes, with the station inside (e.g. NU) lagging the station outside (e.g. KI). There is no change in D component phase across the plasmapause. There is no apparent relationship between the orientation of the major axis of the ellipse and the plasmapause, although there is a longitudinal variation. Only EG on the U.K. meridian does not have the major axis in the NW quadrant, whereas for all the stations in Scandinavia the major axis is in the NE quadrant.
TABLE~.DOMINANTPERIODINTHE H AND D COMPONENTSFOR THET-WOPULSATIONSSHOWNINFIG.~ONDAY~~~,~~~~.NOTE THAT 142.2AND 128.0ARE ADJACENT SPECTRAL ESTIMATESAS ARE 85.3AND 80.0,67.4AND 64.0 AND 160.0 AND 142.2
(b) Day 333 1977 Two Pi 2 pulsations are observed on day 333, at 2302 U.T. and 2322 U.T. (Fig. 4), corresponding to 2302 L.T. and 2322 L.T. on the U.K. meridian and -0030 L.T. and 0050 L.T. on the Scandinavian meridian, note the timing errors at DU and LL. Figure 4(a) shows that both pulsations have similar waveforms from DU to CA on the U.K. meridian, but EG, RY and to a lesser extent FA have more irregular waveforms. Also, the H and D components appear to be different in both events, with the H component having a shorter period in both cases. Comparing the U.K. (Fig. 4a) data with the Scandinavian (Fig. 4b) data, although Pi 2 pulsations occur at the same time, the waveforms are very different on the Scandinavian chain from those observed in the U.K. and Iceland. This is even seen at NU, the lowest latitude station in
present in D. This might suggest that more than one process is present at lower latitudes. Comparing the Scandinavian periods with those from the U.K. there is almost total disagreement in dominant period, unlike the previous event. It should also be noted, however, that the shorter H component dominant period, 8&85 s for event 1 and 71-75 s for event 2, are both present at all the Scandinavian stations, whereas the longer D component dominant period appears only in TR H and D and NU D for event 1 and TR H for event 2. Cold plasma density measurements made on BEE-1 show a well defined plasmapause at L - 4.5 (Fig. 5). The measurements were made in the interval - 203& - 2300 U.T., corresponding to -023@0509 L.T. The relationship between the local time of the Pi 2 pulsations and those of the ISEE- plasma measurements is very similar to the previous event (day 332) and in the same way the plasmapause might be modelled by a line of constant L value from the L.T. of observation, -0330 L.T., to that of the pulsations, 2300-0050 L.T. As only the shorter periods are present in all of the Scandinavian H and D components, complex demodulation results are presented for 83.1 and 74.5 s for events 1 and 2 respectively. For event 1 and T = 83.1 s (Fig. 6) the H amplitude is maximum at EG and KI with a very small secondary maximum at LL. The U.K. amplitude is larger than the Scandinavian. With the plasmapause just poleward of the FA field line, L - 4.5, it is unlikely that the secondary maximum at LL is associated with this feature directly, nor indeed is the maximum at EG. This is associated with the field line on which the Pi 2 pulsation is generated. The D amplitude shows maxima at RY and KI. Comparing H with D, only at OL and NU is D the strongest component and it is
Scandinavia,
which in general might be expected
have similar waveforms
to those
to
seen at LL and ES (Meir-Jedrzejowicz and Southwood, 1979). The differences in the waveforms are highlighted by the power spectra for each event. If the dominant spectral component is considered, Table 2, from FA to CA for event 1 (230&2320 U.T.) the dominant period is the same in the H and D components, except for FA D and CA H. EG tends to show some agreement but RY has a much longer dominant period especially with H component. For event 2, FA to CA again show the same dominant period, but in this case RY agrees with the lower latitude stations and EG differs. Another important feature is that the H component has a shorter dominant period than the D component for both events. The longer period is present in H in both events although at a shorter period in event 2 (128 s as opposed to 142 or 160 s), and the shorter period is
Time interval Station
EG RY FA DU LL ES CA TR KI OL NU
2300-2320 U.T. Period (s) H D
232lF2340 U.T. Period (s) H D
91.4 142.2 80.0 85.3 85.3 85.3 67.4 64.0 67.4 53.3 80.0
160.0 75.3 75.3 75.3 75.3 75.3 75.3 128.0 182.9 49.2 41.3
128.0 160.0 55.7 116.4 128.0 116.4 128.0 106.7 85.3 85.3 64.0
75.3 160.0 160.0 160.0 142.2 160.0 142.2 49.2 106.7 71.1 71.1
Ground-satellite
1000
151
observations of Pi 2 pulsations
-
100 0 ‘E ”
,” C
lo-
l-
.l 2
I
I
I
I
I
I
3
4
5
6
7
0
20.45
23.00
UT
03.00
05.09
LT
MEASURED BY THEISEEFIG. 5. ELECTRONDENSITYvs L PROFILE 333.1977. DAY
333
1977
RELAXATION SOUNDER EXPERIMENT ON DAY
Ev.1
fi amplitude
D amplitude
66
58
[ -i
L
:lIo-_ 6
8
10
12
1
L______ 0
2
4
6
6
10
IiT D phase
-180
12
nT
0
Ellipse Orientation
Ellipticity
+lEo
-1
0
FIG. 6. As FORFIG. 3 FOREVENT1 ON DAY
+1
-90
0
333, 1977.
+90
152
LPSTER and
M.
only at these two stations that the Scandinavian amplitude is larger than the U.K. amplitude. Because of timing errors phase measurements at DU and LL are not included. In the H component phase ES lags CA by - 20” and FA lags ES by 75”. A further difference of -50” in the same direction occurs between FA and RY, but EG leads RY by -90”. On the Scandinavian chain TR and KI are “in phase” but OL differs by -50” and the largest phase difference occurs between NU and OL. It seems that the large phase differences occur across the plasmapause. In the D component phase there is only a similar large phase difference between EG and RY i.e. no phase difference across the plasmapause. The ellipticity shows that there is a change in the sense of polarisation between FA and DU on the U.K. meridian and OL and NU on the Scandinavian meridian. Poleward of this reversal all stations exhibit clockwise rotation and equatorward anticlockwise rotation. This is again the reversal referred to by Fukunishi (1975), Lester and Orr (1981) and others.
DAY
The reversal, however, does occur at a slightly lower L shell than the plasmapause. The ellipse orientation shows similar characteristics to the event on day 332 i.e. only EG on the U.K. meridian has the major axis of polarisation in the NE quadrant. Note that on the Scandinavian chain TR and KI exhibit an almost North-South orientation, but OL and NU show an almost East-West orientation. This confirms the dominance of H at the former two stations and of D at the latter. In discussing the characteristics of the second event (Fig. 7), we shall compare them with those for event 1. In the H amplitude the maxima again occur at EG, KI and LL, a small secondary maximum. In D however the maximum has moved from RY to EG. Also again only OL and NU have D as the dominant magnetic component. In the phase characteristics, there is now a larger difference between FA and RY and a smaller one between FA and ES in the H component. In the D component the phase again remains almost constant across
1977
333
D. Oaa
Ev. 2 D amplitude
H amplitude
62
2 J
0 F w z
501. 0
' 2
' 4
' 6
' 8
' 10
' 12
J 14
IfI 0
1
4
6
6
10
flT H phase
D phase
IlT Ellipticity
Ellipse Orientation
FIG. 7. As FOR FIG. 3 FOR EVENT2 ON DAY 333, 1971.
Ground-satellite
153
observations of Pi 2 pulsations
the plasmapause, but TR differs from KI by - 100”. The ellipticity reversal on the U.K. meridian has moved North of FA but stayed between OL and NU on the Scandinavian chain. This is obviously related to the difference in H component phase seen between events 1 and 2. The orientation of the major axis has
changed, with FA to CA all oriented almost directly North-South, i.e. H is much larger than D, and RY in the NE quadrant and EG in the NW. On the Scandinavian chain TR and KI are now in the NW quadrant, but OL and NU remain almost East-West in direction.
D camp
H corn*
I
4 0 nT
LE -
1
23
,
40
23
I
55
0 IO
I
025
I
043
I
055
J
I
I
1
I 10
23 40
23 55
0 13
FE. 8(a)
1
025
,
I
0 40
0 55
I
IO
154
iI&. LESTERand M
r).
ORR
camp
D camp
I
I
l&OPT
This particular interval is interesting because 3 Pi 2 pulsation events occur close to midnight on day 118, The start times are at 2355 U.T. on day I 18,0030 U.T. and 0045 U.T. on day 119 and the events wiu be known 88 events I, 2 and 3 respective@. Aft three events are seen along both meridians (Fig. 8) although there are again differences between the two meridians e.g. events 2 and 3 are considerably stronger in the D component on Ehe Scandinavian chain. Each. PI 2 pulsation consisted d more than one spectral eomponen$
I
12 0 nT
and considerable variation in the daminant pepetiwrd especially for event 2. For event X the dominant spectral component on the U.K./lcelnnd meridian was at -71 s except at EG where it was at - 150 s. On the ~ca~~a~~a~ meridian the don~~nan~ spectrai ~om~onen~w~ at - 71 s in B, but in ff at TR and KL it was -iIisaadatQLandNUY71s,ForeventZjllD the dominant spectral component ~88 at - 90 s except at EG where it was at - 150 s. For the R component: it varies considerably with EG and KS at - 150 s, Of;, NU_ FA and LE at - 90 s, LL at w 76 s and ES, Y@
Ground-satellite
155
observations of Pi 2 pulsations
and CA at - 55s.The D component was the strongest of the two horizontal components and so the spectral component at -90 s was chosen in the further complex demodulation analysis. For event 3, the dominant component was at -68 s at all stations except EG where it was at - 150 s. Again ISEEcold plasma density measurements (Fig. 9) have been used to estimate the plasmapause position. In this case, however, no unambiguous determination can be found, although a change in plasma density gradient does occur at L - 3.7. The plasma measurements and the Pi 2 pulsations occur very close in L.T., 0144-0403 L.T. for the former and 235550215 L.T. for the latter, but the plasma measurements were made some 8 h in U.T. after the occurrence of the pulsations. The observations were again made in the sector where Chappell et a/. (1970) found the plasmapause position to be determined by the magnetic activity over the preceding 226 h. For both sets of observations this is very similar; K, averages 20 for the 6 h preceding the occurrence of the Pi 2 pulsations and 1+ preceding the plasma density measurements. We suggest that the profile in Fig. 9 is, therefore, similar to the one which was present at the time of the pulsations’ occurrence.
From the results presented in the first two parts of this section it appears that two parameters of the Pi 2 pulsations are sensitive to the position of the plasmapause-El component phase and ellipticity. For the three events in this time interval, therefore, we present only the latitudinal variations of these parameters (Fig. 10). (Note that due to a timing error H component phase information from ES will not be presented.) If we consider first the U.K. H component phase variation, one can see that the difference in phase between FA and LE increases from event to event, while the phase variation from LE to CA is reasonably constant. The phase difference between FA and LE increases from 25” for event 1, to 45” for event 2, to 117” for event 3. Comparing this variation with those seen for the previous 2 events, it is suggested that the plasmapause may be moving radially inwards. The Scandinavian variation in H component phase suggests something similar happening in this meridian, with large variations between KI and OL for events 1 and 3, 82 and 90” respectively, and a rather more gradual change for event 2, from KI to NU, of 77”. The variation of ellipticity with latitude on the U.K. meridian shows a reversal between EC and FA for
0144
0403
FIG.~. ELECTRONDENSINVSLPROFILEMEASIJREDBYTHE
BEE-1
119, 1978.
LT RELAXATIONSOUNDEREXPERIMENTON
DAY
156
M.
LESTERand D. ORR
DAY
118/g
H phase Ev. 2
Ev.1
Ev.3
62
:
54
1 IF a
50 -180
0
f180
Ellipticity
I
58-
54-
L
”,lL !k I
I
50 -1.0
“1 d 0
a.
+1.0
0
-1.0
+ 1.0
-1.0
0
t1.0
FIG.10. VALUES OF H PHASE AND HORIZONTAL ELLIPTICITY FORTHETHREEPi2 PULSATIONS ON DAY 118/9,
1978.
event 1, and between FA and LE for event 3. This is again consistent with a radially inward motion of the plasmapause. The lack of a reversal for event 2 may be explained if the Pi 2 is generated to the South of EG. In this case a reversal in ellipticity would occur at this position with a negative sense of rotation to the South of the generation region (Saito et al., 1976). This in turn would suggest that there is another reversal to the North of FA, which is the one identified with the plasmapause. It is, however, difficult to rationalise the latitudinal variation in ellipticity on the Scandinavian chain for events 1 and 2 in this way and this will be discussed further in the next section. For event 3 there is a reversal between KI and OL. Comparing the above results with the plasma density profile, it seems only event 3 on the U.K. meridian has a reversal which is close to the change in gradient at L - 3.7. The fact that events 1 and 2 do not show this is not unreasonable as some radial motion of
the plasmapause is expected in such magnetic conditions. The difference in position of the reversal between the two meridians may be due to differing positions of the plasmapause and/or plasma gradients. 3. DISCUSSION
The first two comparisons between Pi 2 pulsation characteristics and plasma density measurements, days 332 and 333 1977, suggest that only H component phase and the horizontal ellipticity vary across the plasmapause. The secondary H component amplitude maximum (Stuart and Baransky, 1983) appears to occur well inside the plasmapause. It should be noted that this feature is not well defined for the two pulsations on day 333 or for the three events on days 118/9. On the Scandinavian chain on day 332 there appears a maximum at KI (I_.= 5.31) which is very close to the plasmapause position (L - 5.0).
Ground-.satetiite II has been suggested
observations
(Rostoker and Samson, 1981) the peak power of the Pi 2 pulsation occurs at the equatorward edge of the westward eiectrojet for morning sector events and along the Harang discontinuity for evening sector pulsations. On day 332 a westward eiectrojet is present over both EG and RY and TR and KI at 0045 U.T., just before the Pi 2 onset. The centre of this electrojet is poleward of EG and very close to KI, and the equatorward edge can only be determined as being South of EG (but well to the North of FA) and South of KI (and again well to the North of OL). It seems then that the maxima on both meridians are associated with the electrojet system, although the maximum of Kl may also be associated with the plasmapause. Since it is generally believed that the main source of Pi 2 pulsations is in the electrojet system (Rostoker and Samson, 1981; Samson, 198 1; Pashin et al., 1983fitis more likely to be the former. The two events on day 333 have similar electrojet systems at the time of Pi 2 onset. A westward electrojet is present over Iceland and Scandinavia for both events, the only difference being the movement of the centre of the electrojct North of EG for event 2. This may imply that the equatorward edge is closer to RY but still remaining South of RY. Over Scandinavia the centre is to the North ofKI and the equatorward edge to the South. The electrojet is much stronger, by a factor of -3, during the second event. Qn days 1 IS/9 there is an eastward eiectrojet over Scandinavia during the first event, but over Iceland a westward electrojet is abserved at EG (no horizontal magnetic data are present from RY). For the other two events a westward electrojet is seen, with the centre to the North ofEG and equatorward edge North of RY in both cases. There is no indication that the equatorward edge has moved close to and just South of EG as mentioned earl& in discussing the ellipticity properties for event 2, although this cannot be ruled out. Having discussed the effect of the electrojet system on the amplitude of the Pi 2 pulsation, it should also be pointed out that the electrojets and associated field aligned currents may affect the H component phase (J. 6. Samson, pers. comm., 1982) and the sense of polarisation (Samson, 1981). Indeed Samson (1981) contends that the reversal in the sense of polarisation seen at mid-latitudes can be accounted for in a model of the generation of Pi 2 pulsations through a threedimensional current system induding the electrojet systems, and upward field aligned currents which link through an aurora1 arc in the breakup region and downward field aligned currents at the poleward edge of the electrojet border. This may in part account for
that
of A 2 pulsations
157
the difference in the ellipticity between the two meridians for events 1 and 2 on days ll8/9. Other factors may be influential in the position of polarisation reversal at mid-latitudes. For example three mechanisms have been proposed which would modify the Pi 2 signal away from the aurora1 electrojet; plasmaspheric cavity resonance (Doobov and Mainstone, 1973), surface wave on the plasmapause (Southwood and Stuart, 1979) and line resonance (Stuart et at., i979). With thecoincident plasma density measurements one may be able to decide which of these three mechanisms is the most likely candidate. Figure 11 shows the power amphtude spectra in the W component in the frequency range 5-20 mHz (206 50 s) from the time interval 004~-0105 U.T. on day 332, 1977. Three stations are presented, one from the amoral zone which is close to the generation region of the Pi 2, RY; one from near the plasmapause, FA ; and me which is iocated inside the plasmapause, ES. The dominant period at RY is at 174 s, and the half-power points of this spectral peak are marked on Fig. 10 by the lines A and B. This period is not apparent at FA and although there is some power at ES there is no obvious peak. The dominant period at FA is at - I20 s, and at ES it is at N I12 s. Consider first the spectral peak at - 120 s at FA, which has half-power points indicated by lines C and D (Fig. 11). This spectral peak shows up clearly at ES, and there is also a peak at RY although this is not well defined. Finally let us consider the spectral peak at RY at -78 s, with half-power points E and F (Fig. 11).At FA this spectral peak is not seen but at ES there is a well defined peak. The power spectra at ES is typical of that at the other plasmaspheric stations LE, EL and CA. At RY the power is mainly at Longer periods than those seen to dominate at FA and ES. This suggests that the power in the Pi 2 is being modified at the latitudes corresponding to these stations. Using plasma frequency measurements obtained by the GEOS S-300 experimenters, and sent to us by courtesy of T3r. R. Gendrin, the eingenperiods of the guided poloidal wave, T,, were evaluated at four positions using the eigenvalues computed by Orr and Matthew (I 971) (Table 3). The characteristic periods have been obtained by assuming charge neutrality and that the ions are protons. Also a power law distribution of the form n, = n,fr/r$” where n0 and n, are the proton number of densities at r0 and r respectively and m = 3 has been assumed. Chen and Hasegawa (1974) consider the characteristic period of a surface wave that may be generated at a plasma density discontinuity such as the plasmapause. The surface wave will travel over the boundary at a speed which is some average of the Alfven velocity
M. LESTERand D. ORR
158
H Comp.
0.11
5
FIG. 11. H
COMPONENT
I
I
I
10
15
20
mHz
POWERSPECTRA FORTHREESTATIONS FROMTHETIMEINTERVAL 004&0105 U.T. DAY 332,1977, IN THEFREQUENCY RANGE5-20 mHz (200-50 s).
See text for full details.
on the two sides of the boundary. The period of the surface wave, T,, will be given approximately by the longest period within the plasmapause in the guided poloidal mode divided by fi (A. Hasegawa, personal communication). From Table 2 this is seen to be about 109 s, in encouraging agreement with the observed dominant period at the plasmapause and within the plasmasphere. This together with the obvious change in spectra
between RY and FA suggest that the surface wave is dominating at FA and although attenuated inside the plasmapause is the dominant component. Also the component at -78 s, seen at RY and also at ES is swamped by this surface wave at FA. The fact, however, that RY and ES see similar periods suggest that this period is not necessarily related to a surface wave and at ES may simply be the response of “midlatitude” plasma to a forcing source field at higher
Ground-satellite
159
observations of Pi 2 pulsations
TABLE 3. CALCULATED CHARACTERISTIC PERIODS, TI (s)USING THE RESULTS OF ORR MATTHEW(~~~~)ANDTHECHARACTERIS~CSURFACEWAVE,T,(S),USINGTHEELECTRONDENSI~ MEASUREMENTSIN FIG. 2
Position
U.T.
L.T.
L
R/R,
f, &Hz)
n, (cm- 3,
Tl (s)
1
0100
2 3 4
0105 0110 0130
0431 0441 0447 0503
4.98 5.12 5.26 5.71
4.70 4.83 4.95 5.34
60 55 35 20
45 38 15 5
150 154 107 86
latitudes. No other evidence to support this, however has been found from the complex demodulation analysis. Making the same assumptions as before, the characteristic periods have been obtained for the plasma density profile for day 333(Table 4). The surface wave period here is - 170 s. This period is not seen at any of the stations inside the plasmasphere and a comparable period is only seen at RY in the D component at 160 s. It is interesting to note that the dominant H component periods for both events 1 and 2 are close to the second harmonic of the characteristic surface wave. The other difference for the events on day 333 is the apparent decoupling of H and D at stations within the plasmasphere. This suggests that there may have been more than one type of mechanism present for these events. The characteristic periods for the guided poloidal mode based on the plasma density measurements made on day 119 (Table 5) show that there is a decrease in the characteristic period at L - 3.7 which is the position where there is a change in the gradient of the plasma density (Fig. 9). This change in gradient may be
TABLE 4. As FOR TABLE 3 BUT USING THE ELEcTRONDENSlTY MEASUREMENTSIN Fic.5
n, (cm - “)
L
T (4
1000
3.29 4.00 4.25 4.42 4.54 4.65
135 221 237 219 167 58
-
510 348 215 100 10
T (4
167
TABLE 5. As FOR TABLE 3 BUT USING THE ELECTRONDENSITYMEASUREMENTSIN FIG.9 n, (cm 1000
350 190 100
“)
L
TI 6)
2.7 3.6 3.7 4.0
58 114 100 98
x (4
81 _
AND
T, (s) 109
able to support a surface wave which would have the characteristic period of - 81 s. This is in encouraging agreement with the dominant periods of the three events which are in the range 68-93 s, despite the variation in spectral content especially in event 2.
4.CONCLUSlONS
We have attempted to correlate satellite plasma density measurements with the latitudinal variations of the amplitude, phase and polarisation parameters of Pi 2 pulsations measured on two meridional chains of magnetometers separated by - 1s h in time. Several conclusions can be made : (a) There appears to be a close relationship between the position of the plasmapause and the position of an ellipticity reversal. (b) There is also a variation in H component phase across the plasmapause position which is not seen in the D component phase. (c) Characteristic surface wave periods calculated using the cold plasma density measurements appear to be present in the spectra of stations inside the plasmapause for four of the six Pi 2 events. (d) At times there is some decoupling of H and D so that each magnetic component will have a different dominant period. (e) On occasions there is no high degree of coherency between stations inside the plasmapause. Of the three mechanisms mentioned earlier, the surface wave on the plasmapause appears to play the dominant role at mid-latitudes. A cavity resonance would typically give periods in the range 4&50 s for the plasma measurements presented here and there is not a sufficiently strong secondary amplitude maximum for the line resonance to be considered. Also for a line resonance an ellipticity reversal should occur close to the secondary maximum which is not the case. Events do occur, such as those on Day 333‘where the calculated surface wave periods do not agree with the observed dominant periods and such cases therefore may not be due to the surface wave mechanism. The important feature in this case is the lack of coherency
160
M. LESTER and
between stations in the U.K. and those in Scandinavia which may be a means of identifying such cases when plasma density measurements are unavailable. It is also possible that factors other than the plasmapause play an important role at mid-latitudes (Samson, 1981). This would certainly be true close to the meridian of the source of the Pi 2 pulsations and during extremely disturbed conditions and strong electrojets. The role of the electrojets and field aligned currents need to be modelled further and the plasmapause needs to be taken into account in the model. Also further experimental work with a closely spaced magnetometer array, covering both mid-latitudes and the aurora1 regions with coincident satellite measurements of the plasma density needs to be carried out.
Acknowledgements-We would like lo acknowledge the help of all the people who have operated magnetometer stations since the network was set up ; in particular the observers at the 13 sites used in this paper. We would also like to acknowledge the work of Dr W. F. Stuart who co-ordinated the network. We thank Drs R. Gendrin and J. Etcheto for supplying the plasma density measurements. Thanks go to Drs M. R. Warner and C. A. Green for helpful discussions. One of us (M.L.) was supported by the SERC on research grant SD 02462.
REFERENCES Beamish, D., Hanson, H. W. and Webb, D. C. (1979). Complex demodulation applied to Pi 2 geomagnetic pulsations. Geophys. J. R. astr. Sot. 58,471. Carpenter, D. L. (1966). Whistler studies of the plasmapause in the magnetosphere. 1. Temporal variations in the position of the knee and some evidence on plasma motions near the knee. J. geophys. Res. 71,693. Chaonell. C. R.. Harris. K. K. and SharD, G. W. (1970). A study of the influence bf magnetic activiiy on the l&at&n of the plasmapause as measured by OGO 5. J. geophys. Res. 75, 50. Chen, L. and Hasegawa, A. (1974). A theory of long period magnetic pulsations. 2. Impulse and excitation of surface eigenmodes. J. geophys. Res. 79, 1032. Doobov, A. L. and Mainstone, J. S. (1973). Investigations of
D. ORR
Pi 2 micropulsationsRelevance of observations generation theories. Planet.Space Sci. 21,731.
to
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